[fpc-devel] Interface scope incompatibility with Delphi

Thomas Schatzl tom_at_work at gmx.at
Fri Nov 12 11:14:54 CET 2010


On Fri, 12 Nov 2010 00:46:43 +0100 (CET), Dimitri Smits
<smitco at telenet.be> wrote:
> it seems that the stackvariables are NOT unloaded in the correct order
> (ie: reverse order of declaration).
> It shouldn't matter in what order you fill them, but in the order they
> declared. (like good practice in constructor vs destructor, but then in
> stack variables setup/cleanup)

Imho the compiler should generally be free to reorder/remove/reuse/add
temporary variables on the stack as it sees fit. What you experience is
only an artifact of the current implementation.
Making assumptions about exact stack layout (in particular of local
variables) beyond what is specified by the calling convention is generally
not a good idea.

> ---8<-----------8<-----------8<-----------8<-----------8<-----------
> [... snipped use of interface hacks to implement scoped memory...]
> should I file (2/a) bug report(s)? or is there no point in doing so
> they won't be fixed either?

Imo: Jonas already stated, the time of the release for interfaces is not
guaranteed - afaik the docs/specs only state that they "will be released if
the reference count gets zero" (I have not read the docs for the exact
So what if the runtime system internally keeps a reference for en masse
garbage collection using a background thread? (I.e. the refcount starting
at one. This is a totally valid use case for interfaces, and which btw,
also makes the "stack ordering" issue a moot point too.

Or, this interface memory management may be implemented using tracing
garbage collection in some future target (LLVM, JVM, CLR, ...) the same

What would be interesting would be how garbage-collected Delphis handle
this, can anyone help? Other than that, there is imo nothing else to say
than that Pascal/Delphi(*) does not have explicitly scoped lifetimes with
automatic destructor call for classes/objects at the moment.

(*) maybe newer Delphis have the C# "using" keyword or something similar
(maybe "with" recycling?). I would be interested to hear about that.


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